1. Field of the Invention
This invention relates to the field of communication data, and more particularly to methods and apparatus for determining symbol estimates and reliability weights, and for pre-selecting symbols based upon reliability criteria.
2. Description of Related Art
Assessing the reliability of received data is important in communication systems. For example, data reliability assessment is important when applying decision-assistance to demodulation tasks, such as carrier phase tracking and equalization. This is particularly true when the demodulator precedes a turbo decoder, and the system is operating at very low symbol-to-noise ratios. In this case the symbol error rates can become so large that the demodulation process is severely degraded, and consequently the decision-directed demodulation process becomes undesirable.
Under such conditions, symbol pre-selection becomes desirable. When a symbol decision is highly likely to be error-free, it is desirable to use the associated datum for further processing. When a symbol decision is unlikely to be error-free, it is desirable not to use the associated datum. It is desirable to avoid using errored data because harm caused by bad data may exceed the benefit provided by good data.
Decision-assisted data processing can be enhanced if the reliability assessment of symbol data is provided in a soft format, such as a linear probability with a range of values between 0 and 1, inclusive. This enables the decision-assisted process to “weight”, or equivalently, to “pre-select” symbol data prior to subsequent processing.
Therefore, a need exists for a method and apparatus for providing soft reliability determination of received data, and for pre-selecting symbol data based on reliability criteria.